Typical radiometric receivers may use a technique proposed by Robert Dicke, in which the receiver is rapidly switched between a known reference source and the scene being measured. This may allow for compensation of temporal variations in the gain of the low noise amplifier that is necessary due to the low signal levels being measured.
One obstacle to imaging in the millimeter and submillimeter wavelength bands, for example, is that the longer wavelengths of such bands may require larger apertures to achieve the resolutions typically desired in surveillance applications. As a result, lens-based focal plane systems may require large-aperture optics, which may severely limit the minimum achievable volume and weight of such systems. One approach to overcome this would be to use a distributed aperture detection scheme, in which the effective aperture size can be increased without the associated volumetric increase in imager size. A sparse, or distributed, aperture imaging system may use multiple (say, N) detectors to capture the phase and amplitude of the scene/object being imaged at each point. However, such systems, sometimes used, e.g., by the radio astronomy community, may require high frequency (30-300 GHz) signal routing and down conversion as well as large correlator banks.
In such systems, again, compensation for temporal variations in the gains of the low noise amplifiers of the receivers may be beneficial, e.g., due to the low signal levels being measured. Using a Dicke-type system for the multiple detector signals may be possible, but this would further increase the size and weight of the equipment and may also introduce additional difficulties, e.g., complexity and/or insertion losses.